The present invention generally relates to ac power control, and particularly relates to synchronization of ac power inverters.
Increasing use of localized power sources in residential and industrial settings coincides with the increasing desire to exploit potentially cheaper, cleaner, or more reliable alternative energy sources. Historically, individuals and businesses relied on central generation stations with power transmission and distribution networks, i.e., the traditional “grid,” almost exclusively but the developing trend is toward a number of smaller generating units, distributed geographically close to the loads. Local power generation using power sources such as photovoltaic systems, fuel cells, etc., also is becoming a viable option for businesses and homeowners. These local power generating systems can be configured as stand-alone systems that operate as the exclusive source of power for their local loads or they can be interconnected with the grid.
More commonly, these local power generating systems are configured as grid-connected systems that provide an opportunity for co-generation wherein power can be taken from the grid or provided to the grid. A grid-connected system allows a local load to remain connected both to the local system and to the power grid. In a typical setup, a local load is connected in parallel to an ac power inverter and to a utility grid. The load can be powered solely by the inverter, by the grid, or can receive a portion of its power from both.
Synchronization of the inverter to the grid is a prerequisite for changing from stand-alone mode to grid-connected mode. That is, the inverter's voltage must be matched in amplitude and phase to the voltage of the grid before electrically connecting the inverter to the grid.